An optical communication system includes optical transmission cables for transmitting optical signals. An advanced commercial optical communication system utilizes numerous signal channels over a wide bandwidth in each optical fiber in a wavelength division multiplexed (WDM) transmission cable system to achieve high speed and high capacity optical signal transmission. Light propagating within an optical fiber, particularly in ultra long-haul WDM transmission cable systems, undergoes chromatic dispersion, which causes the light pulse to be delayed within the optical fiber, and optical nonlinear effects, which impair the transmission performance. Thus, the optical fiber transmission lines and cables used in such systems have been designed with different types of optical fibers in accordance with dispersion maps to compensate for dispersion and to mitigate the effects of fiber nonlinearities.
A typical WDM transmission cable system designed to a dispersion map comprises transmission cable spans and compensation cable spans. As used herein, “cable span” refers to the cable section between two repeaters, or between a transmitter and the repeater closest to the transmitter, or between a receiver and the repeater closest to the receiver. To satisfy the transmission performance requirements of a WDM transmission cable system, the optical transmission lines in each cable span preferably have desirable transmission characteristic, e.g., to manage the chromatic dispersion and loss across the transmission bandwidth over the span. The transmission lines should also be cost effective for deployment within the cable spans.
One example of an optical transmission cable is disclosed in U.S. Pat. No. 6,496,629, which is fully incorporated here in by reference. Referring to FIG. 1, this type of optical transmission cable 10 generally includes multiple optical fiber transmission lines 12 having optical fibers embedded in a thixotropic water blocking gel and extending through a tube 16 made of a polymeric material. First and second layers of ultra-high strength steel wires 18a, 18b extend along and around the tube 16 together with a polymeric water-blocking material. A copper sheath 20 surrounds the steel wires 18b and polyethylene insulation 22 surrounds the copper sheath 20. Referring to FIG. 2, the cable 10 is typically constructed from multiple pairs of optical fibers 24a, 24b. The pairs of optical fibers 24a, 24b become pairs of optical fiber transmission lines 12a, 12b in the cable 10. The optical fiber transmission lines 12a, 12b transmit optical signals in opposite transmission directions.
Examples of dispersion-managed optical fiber transmission lines and cables are disclosed in U.S. Pat. Nos. 5,894,537; 6,301,419; and 6,477,306 and U.S. Patent Application Publication Nos. 2002/0181076 and 2002/0048439, all of which are fully incorporated herein by reference. These dispersion-managed transmission lines typically combine sections of constituent optical fibers with different dispersion characteristics to manage dispersion and/or to suppress nonlinear optical effects. For example, a transmission line can include one or more positive dispersion constituent fibers (called P-type fibers) on the upstream transmission direction and one or more negative dispersion constituent fibers (called N-type fibers) on the downstream transmission direction.
There are several common problems associated with existing dispersion-managed transmission lines. Unpredictable trimming on both ends of the transmission lines during the cabling process often varies the total lengths of the P-type fibers and the N-type fibers. As a result, the chromatic dispersion properties of the transmission line may be altered.
Existing transmission lines also present problems when a length of transmission line needs to be removed and/or replaced to repair a fiber defect or a break in either a fiber or cable during cabling process or cable span assembly process. A repair with a replacement cable portion typically involves more than one splice, which complicates the cabling process and cable span assembly and usually increases the loss of the transmission line. If a repair occurs in an area with N-type fibers, in particular, the effect on the loss characteristics of the transmission line may be significant. The loss for each cable span should preferably be brought as close as possible to the repeater gain to achieve optimized performance across the transmission band in a WDM transmission cable system.
The optical fiber transmission lines and cables mentioned above also do not allow easy identification of fiber layout and transmission direction within a cable. If it is not easy to identify the fiber layout and transmission direction in a dispersion-managed transmission cable, system assembly, installation, and maintenance can be difficult, time-consuming, and not cost effective.
One optical transmission line and cable that has attempted to address one of these problems is disclosed in U.S. Pat. No. 6,421,484, which is fully incorporated herein by reference. This transmission line and cable attempt to maintain a desirable mean transmission characteristic as a whole regardless of the fluctuation in total length due to unpredictable trimming on the end portions. The main transmission line comprises one or more P-type fibers of a relatively larger mode field diameter and one or more N-type fibers of a smaller mode field diameter. Separate optical fibers having a chromatic dispersion equal to or less than the absolute value of the mean chromatic dispersion of the main transmission line are used as length extensions on both ends of the main transmission line to minimize the impact of the unpredictable length trimmings on the mean chromatic dispersion of the transmission line. The optical fibers for the length extensions also have their mode field diameters selected to optimize the fusion splices to the main transmission line and the repeater tail.
The transmission line and cable disclosed in U.S. Pat. No. 6,421,484 suffers from some drawbacks. Providing length extensions using optical fiber types different from those in the main transmission line degrades system performance and complicates the assembly process. In particular, the use of different fiber types to provide length extensions involves extra steps during cable production (e.g., extra fusion splices) and requires extra cable types for system maintenance and repair. The added length also may not be able to compensate for the deterioration in transmission characteristics, such as chromatic dispersion, due to the removal of fiber or cable during the cabling process and cable span assembly process without inserting replacement fibers or cables. The hundreds of meters of length extension in front of the optical fibers of positive dispersion and larger mode field diameter may cumulatively impact the efficiency of nonlinearity suppression due to the smaller mode field diameter of the fibers used for the length extension, especially in an ultra long haul and ultra high capacity system.
The length extensions with different fiber types also make it more difficult to inspect the main transmission line during the cabling process. Furthermore, even though these length extensions introduce only a fraction of cumulative dispersion in their corresponding spans, in aggregate they might cause significant cumulative dispersion and dispersion slope in an ultra long haul system with hundreds of spans. Such cumulative dispersion and dispersion slope may be difficult to compensate simultaneously.
U.S. Pat. No. 6,421,484 also discloses that the main transmission line is color coded uniformly together with the length extension on the negative dispersion fiber end and the length extension at the positive dispersion end is color coded with a different color to indicate the transmission direction. This color scheme has disadvantages. Because each transmission line is color coded uniformly, the cable including these transmission lines loses its symmetry and must be marked to indicate orientation. This color scheme is also not cost effective for the repair of an installed system.
Accordingly, there is a need for a transmission line and optical cable, and a method of making a transmission line and cable, that address the problems discussed above.